1. Field of the Invention
The present invention relates to a stacked piezoelectric device adapted to extend and contract along the height of the stack upon energization and a method of fabrication thereof.
2. Description of the Related Art
The following-described configuration of the conventional stacked piezoelectric device is well known.
As shown in FIG. 14, a stacked piezoelectric device 9 comprises a piezoelectric stack formed of piezoelectric layers 931 and 932, internal electrode layers 941 and 942 alternately formed between the piezoelectric layers 931 and 932 with alternate positive and negative voltages applicable to the piezoelectric layers 931 and 932, and a pair of side electrodes 91 formed on the sides 901 and 902 of the piezoelectric stack.
In the piezoelectric stack, the internal electrode layers 941 are arranged to be exposed to the side 901, while the internal electrode layers 942 are arranged to be exposed to the other side 902.
A side electrode 91 is formed on each of the sides 901 and 902 of the piezoelectric stack in such a manner as to energize the ends of the internal electrode layers 941 and 942 exposed thereto. The other side electrode electrically connected with the ends of the internal electrode layers 942 is hidden and therefore not visible in FIG. 14.
The conventional stacked piezoelectric device 9 shown above has the problem that cracking tends to occur in the N portion of FIG. 15 in the direction toward the side 901 from the end of the internal electrode layer 941 (A similar problem is also affects the other side 902, not shown).
As shown in FIG. 15, the end portion 944 of the internal electrode layer 942 not exposed to the side 901 has a progressively tapered section in the piezoelectric stack. The end portion 943 of the internal electrode layer 941 is exposed to the side 901.
Though not shown, the end portion of the internal electrode layer 942 is exposed to the side 902, while the end of the internal electrode layer 941 is not exposed to the side 902 of the piezoelectric stack but located within the piezoelectric stack with the section thereof progressively tapered.
As a result, the piezoelectric layers 931 and 932 are divided into a portion M sandwiched between the internal electrode layer 941 and the internal electrode layer 942, and a portion N in contact with either the internal electrode layer 941 or 942.
Upon application of a voltage from the internal electrode layers 941 and 942 to the piezoelectric layers 931 and 932, the portion M sandwiched between the internal electrode layers 941 and 942 can be displaced along the height of the stack. The portion N, however, cannot be displaced, as it is in contact with only one of the internal electrode layers 941 and 942.
Stress develops in the portion L indicated by dashed line in FIG. 15 constituting the boundary between the portions M and N which is in contact with the portion displaced and the portion not displaced.
Thus, the piezoelectric stack may be damaged by cracking occurring from the end portion 942 toward the side 901.
This damage occurs especially after the stacked piezoelectric device is used for a long time or in a harsh operating environment, and has been a major cause of device deterioration.
Also, in the conventional stacked piezoelectric device 9, the internal electrode layers 941 and 942 are configured on a part of the piezoelectric layers 931 and 932. For this reason, a complicated and troublesome process control is required to form the internal electrode layers 941 and 942 of a predetermined size at exact positions on the piezoelectric layers 931 and 932 at the time of manufacture, and therefore simplification of the process control is desirable.
In order to obviate this problem, a method has been proposed to form each internal electrode layer over the entire surface of the corresponding piezoelectric layer.
In this configuration, the internal electrode layers and the piezoelectric layers have substantially the same area. Also, each side electrode is configured in such a manner that the ends of alternate ones of the internal electrode layers are covered with an insulative portion, and the other ends are electrically connected by a conductive portion covering the insulative portions, so that each piezoelectric layer is sandwiched between internal electrode layers of different polarities.
This configuration, however, still has the problem of durability of the piezoelectric device.
Specifically, in view of the fact that the stacked piezoelectric device is displaced along the height of the stack, stress acts on the side electrodes along the height of the stack. Since the conductive portions are formed only at the required points, the mechanical strength of the conductive portions is so low that they can easily become separated from the internal electrode layers.
As described above, with the configuration having conductive portions to energize the internal electrode layers formed over the entire surface of the piezoelectric layers, it is difficult to produce a piezoelectric device high in durability.
The present invention has been achieved in view of the problems of the prior art described above, and the object thereof is to provide a stacked piezoelectric device having a high durability and a method of fabrication thereof with a simplified production process control.
According to a first aspect of the invention, there is provided a stacked piezoelectric device comprising:
a piezoelectric stack including piezoelectric layers adapted to extend and contract in accordance with a voltage applied thereto and internal electrode layers for supplying the applied voltage, the piezoelectric layers and the internal electrode layers being stacked alternately with each other; and
a first side electrode arranged on one side of the piezoelectric stack and a second side electrode arranged on the other side of the piezoelectric stack, the side electrodes being so configured that the internal electrode layers adjacent to each other with a piezoelectric layer therebetween are energized to different polarities;
wherein the piezoelectric layers and the internal electrode layers are configured to have substantially the same area;
wherein said internal electrode layers have the ends thereof exposed to the sides of the piezoelectric stack;
wherein the first side electrode has a first insulating portion formed to cover each of the ends of alternate ones of the internal electrode layers exposed to one side of the piezoelectric stack, a first conductive portion being arranged over the first insulating portions along the height of the piezoelectric stack;
wherein the first side electrode energizes alternate ones of the internal electrode layers;
wherein the second side electrode has a second insulating portion formed to cover each of the ends of alternate ones of the internal electrode layers not formed with the first insulating portion on the other side of the piezoelectric stack, a second conductive portion being arranged over the second insulating portions along the height of the piezoelectric stack;
wherein the second side electrode energizes alternate ones of the internal electrode layers;
wherein the first and second insulating portions are formed of an insulative resin; and
the first and second conductive portions are formed of a conductive resin.
The most notable feature of the present invention is that the piezoelectric layers and the internal electrode layers are configured to have substantially the same area, that each internal electrode layer has an end thereof exposed to a side of the piezoelectric stack, that the first and second side electrodes include the first and second insulating portions, respectively, covering the ends of the internal electrode layers and the first and second conductive portions arranged on the first and second insulating portions, respectively, and that the first and second insulating portions are formed of an insulative resin while the first and second conductive portions are formed of a conductive resin.
The operation of the present invention will now be explained.
The fact that the piezoelectric layers and the internal electrode layers have substantially the same area, i.e. that the internal electrode layers are formed over the entire surface of the piezoelectric layers (FIG. 2) eliminates the need to control the area in which the internal electrode layers are formed on the piezoelectric layers.
This facilitates the process control for fabricating the piezoelectric device.
Also, both the insulative portions and the conductive portions are formed of a resin and have a high elasticity. As a result, the stacked piezoelectric device is less likely to be damaged or broken by stress caused or derived by displacement.
Further, in view of the fact that the piezoelectric layers and the internal electrode layers have the same area, each of the piezoelectric layers is sandwiched between the adjacent internal electrode layers over the entire surface thereof. Therefore, the piezoelectric layers do not have a portion M which tends to be displaced, or a portion N which does not tend to be displaced, and therefore the piezoelectric layers are less likely to be damaged by displacement.
In this way, a stacked piezoelectric device having high durability can be produced. Also, the stacked piezoelectric device can be used very reliably for a long time repeatedly or in a harsh operating environment.
Further, the durability can be improved of a high-performance piezoelectric device having a large displacement along the height of the stack.
As described above, according to this invention, a stacked piezoelectric device is provided which has high durability and the process control for manufacture of which is simplified.
Since the stacked piezoelectric device according to this invention can be repeatedly used in a harsh operating environment and maintain high durability against a large displacement, it is suitable as a drive source of an injector.
If the piezoelectric stack is parallelepipedal as shown in FIG. 1, it can include one side and the opposed side of the parallelepiped on which the first side electrode and the second side electrode can be formed, respectively (see FIG. 9).
In the case of a piezoelectric stack having curved sides, on the other hand, the first side electrode having an appropriate peripheral width is formed on the outer side, and the second side electrode having an appropriate width is formed at a peripherally distant position from the first side electrode (see FIG. 10).
The sides may be present at the so-called corners (see FIG. 10).
According to the second aspect of the invention, the insulative resin is preferably formed of a selected one of epoxy resin, polyimide resin, silicon resin, fluoro resin, urethane resin, acrylic resin, nylon resin and polyester resin.
These resins have superior elasticity, and therefore the first and second side electrodes are not easily damaged or broken due to stress applied thereto by displacement of the stacked piezoelectric device.
Also, it is especially desirable to use epoxy resin, polyimide resin, silicon resin or fluoro resin. All of these resins have not only the elasticity described above, but also superior heat resistance, oil resistance and chemical resistance. It is therefore possible to produce a stacked piezoelectric device which does not easily degenerate even in a harsh operation environment.
According to the third aspect of the invention, the conductive resin contains a metal material and a resin material, the metal material preferably being formed of at least a selected one of silver, gold, copper, nickel, a silver-palladium compound, carbon and tin.
The above-mentioned metal material formed of at least selected one of silver, gold, copper, nickel, a silver-palladium compound, carbon and tin has high conductivity, which ensures conduction with the internal electrodes, thereby making it possible to produce a stacked piezoelectric device having high performance.
In particular, gold, silver and a silver-palladium compound have extremely small migration, resulting in improved humidity resistance of the stacked piezoelectric device.
According to the fourth aspect of the invention, the conductive resin contains a metal material and a resin material, the resin material preferably being formed of at least a selected one of epoxy resin, polyimide resin, silicon resin, fluoro resin, urethane resin, acrylic resin, nylon resin and polyester resin.
These resins have superior elasticity, and therefore the first and second side electrodes are not easily damaged or broken due to stress applied thereto from displacement of the stacked piezoelectric device.
Also, it is especially desirable to use epoxy resin, polyimide resin, silicon resin or fluoro resin. All of these resins have not only the elasticity described above, but are also have superior heat resistance, oil resistance and chemical resistance. It is therefore possible to produce a stacked piezoelectric device which does not easily degenerate even in a harsh operation environment.
According to the fifth aspect of the invention, the amount of the metal material added to the conductive resin is preferably 50 to 90% by weight.
As a result, the first and second side electrodes have sufficient conductivity. If the amount of the metal material added is less than 50% by weight, the probability of the metal materials being brought into contact with each other is greatly reduced, with the result that there may not be conductivity between the first and second electrodes.
On the other hand, in the case where the amount added is larger than 90% by weight, the amount of the resin material is insufficient and mutual contact between the metal materials may become unstable. In other words, the resin material ensures the contact of the metal materials, and therefore in the case where the amount of the resin material is too small, the conductivity of the first and second side electrodes may be unstable.
The amount of the metal material added is defined as the amount of the metal material contained in the conductive resin which is assumed to be 100% by weight. In other words, a preferable range according to this aspect is 50% by weight of the metal material and 50% by weight of the resin material to 90% by weight of the metal material and 10% by weight of the resin material.
According to the sixth aspect of the invention, the modulus of elasticity of the insulative resin and the conductive resin is preferably 0.1 MPa to 40 GPa.
As a result, when the stacked piezoelectric device is in operation, the first and second side electrodes are not easily cracked and therefore a device having high durability can be produced.
In the case where the modulus of elasticity is less than 0.1 MPa, the mutual contact between the metal materials contained in the conductive resin may become unstable. Thus, the conductivity of the first and second side electrodes may also become unstable.
On the other hand, in the case where the modulus of elasticity is larger than 40 GPa, the first and second side electrodes cannot be expanded or contracted according to the expansion/contraction of the piezoelectric stack making up the piezoelectric device in operation, so that the side electrodes may develop cracking and their conductivity is liable to be reduced.
Preferably, in order to stabilize the connection between the metal materials, the modulus of elasticity of the insulative resin and the conductive resin is 1 MPa.
According to the seventh aspect of the invention, the specific electric resistance of the insulative resin is preferably not less than 108 xcexa9/cm.
As a result, insulation can be ensured in the first and second insulating portions. On the other hand, in the case where the specific electric resistance is less than 108 xcexa9/cm, the insulation characteristics of the first and second insulating portions are so low that it is difficult to apply positive and negative power to the two sides of the piezoelectric layer, and thus the performance of the stacked piezoelectric device is liable to be reduced, since in this case, the higher the specific electric resistance, the better.
According to the eighth aspect of the invention, the specific electric resistance of the conductive resin is preferably not more than 10xe2x88x921 xcexa9/cm.
As a result, conduction can be ensured in the first and second conducting portions without fail. On the other hand, in the case where the specific electric resistance is larger than 10xe2x88x921 xcexa9/cm, the conduction characteristic of the first and second conducting portions is so low that it may become difficult to apply a voltage to the piezoelectric layers through the first and second side electrodes.
In this particular case, the smaller the specific electric resistance, the better.
According to the tenth aspect of the invention, the piezoelectric device preferably comprises first and second lead-out electrodes electrically connected to the first and second side electrodes.
As a result, a power supply or the like can be easily connected to the first and second side electrodes.
According to the eleventh aspect of the invention, the piezoelectric device preferably comprises first and second lead-out electrodes at least partly buried in the first and second side electrodes and connected to the conductive resin when the latter is formed.
As a result, the side electrodes are formed at the same time that the first and second lead-out electrodes are coupled, thereby simplifying the process control and reducing the number of steps for fabricating a stacked piezoelectric device according to this aspect.
According to the twelfth aspect of the invention, the electric conduction of the first and second lead-out electrodes is preferably ensured from one end portion along the height of the piezoelectric stack to the other end portion thereof.
As a result, the first and second lead-out electrodes can be connected more firmly to the stacked piezoelectric device. Also, even in the case where part of the lead-out electrodes becomes separated, the fact that electric conduction is ensured from the top to the bottom of the piezoelectric stack results in high reliability.
According to the thirteenth aspect of the invention, the first and second lead-out electrodes are preferably corrugated, slitted, combed or meshed in shape.
The first and second lead-out electrodes having the above-mentioned shapes have high flexibility, and can easily absorb displacement. Therefore, the first and the second lead-out electrodes do not easily come off or lose contact from the piezoelectric stack when the piezoelectric device is expanded or contracted along the height of the stack, thereby improving the reliability of the piezoelectric device.
According to the fourteenth aspect of the invention, a thin electrode film is preferably interposed between the first and second conducting portions and the side surfaces of the piezoelectric stack (see FIG. 13).
As a result, the close contact between the first and second conducting portions and the piezoelectric stack and the electric conductivity with the internal electrode layers can be improved.
A thin conductive film is formed on a part or the whole of the contact surface between the first and second conducting portions and the piezoelectric stack. Of course, the thin electrode film can also be formed after the first and second insulating portions. The thin electrode film may be interposed between the first and second insulating portions and the first and second conducting portions. The absence of the thin electrode film is desirable up to the upper and lower ends of the piezoelectric stack, in order to ensure insulation.
According to the fifteenth aspect of the invention, the thin electrode film is preferably a plated film or a vapor deposited film.
As a result, a uniform thin electrode film in close contact with the piezoelectric stack can be formed.
The invention according to the sixteenth aspect relates to a method of fabricating a stacked piezoelectric device as described in claims 1 to 15, and characterized in that after forming the insulative resin layers on the side surfaces of the piezoelectric stack, the insulative resin layers are removed from alternate ones of the piezoelectric layers thereby to form an insulating portion.
As a result, the process control such as for the insulation distance is eliminated, and the subsequent process control can be facilitated.
According to the seventeenth aspect of the invention, the insulative resin layer is preferably formed by a selected one of the ink jet method and the printing method.
According to these methods, the thickness and width and the coating distance of the insulative resin layers are easily controlled, and therefore an insulative resin layer of a predetermined shape can be accurately produced.
According to the eighteenth aspect of the invention, the insulative resin layers are preferably removed by the laser or photolithography method.
According to these methods, fine control of partial removal can be easily effected so that an insulating resin layer of precise size and shape can be attained.